In recent years, a technique of separating, in a picture image of a subject, a diffuse reflection component from a specular reflection component in reflected light from the subject has been developed. This is because many of image processing algorithms are predicated on diffuse reflection, and specular reflection may cause decrease in performance of image processing algorithms. Patent Literature 1 discloses an example of a technique to separate the above two reflection components.
Patent Literature 1 discloses an image processing method including the following steps (1) through (5).
(1) Under arbitrary illumination, a plurality of picture images of a subject are taken with a camera through polarizing elements whose principal axis directions are different from each other.
(2) For each of pixels in a pixel group in which specular reflection occurs in the plurality of picture images, an incidence plane is identified based on a normal vector and an eye vector of the subject.
(3) For each of the pixels, an incident angle is identified based on the normal vector and the eye vector of the subject.
(4) A pixel set is formed by clustering pixels which have similar incidence planes and also similar incident angles.
(5) In the pixel set, stochastic independence between a diffuse reflection component and a specular reflection component is presumed, and those reflection components are separated from each other.
According to the image processing method disclosed in Patent Literature 1, the diffuse reflection component can be separated from the specular reflection component even under a general illumination environment.